Juan Manuel Espíndola

Field observations of pristine block- and ash-flow deposits emplaced April 16–17, 1991 at Volcán de Colima, Mexico

Abstract For the first time since the effusive activity of 1981–1982, Volcan de Colima extruded scoriaceous andesitic lava on March 1 1991. This followed a period of increasing seismicity, summit dome deformation, and fumarolic activity that began in February of 1991. Growth of the new lava dome proceeded until April 16 at 16:00 hrs (local time, GMT -6 hours), when a series of gravitational failures of the upper edifice and the new lava dome-flow triggered the emplacement of successive small-volume, non-glowing, block- and ash-flows to a distance of 4 km within the upper reaches of Barranca El Cordoban, south-southwest of the present summit. Aerial photos taken at the time of the event clearly show the lack of explosions at the summit dome. Efficient elutriation of the finest silt and clay-sized fraction from the main body of the avalanching material produced large reddish, yellowish, and grayish ash clouds which rose to an altitude of 1.5–2.5 km for several hours and were carried southeast to northeast by predominant winds. Vegetation in the path of the debris was totally stripped, sand-blasted, and bent in the sense of flowage but was not burnt. No fatalities were reported. We present a summary of our field observations of the pristine deposits which we studied prior the rainy season, on April 20–27 between 2,200 and 2,500 m elevation.

Holocene plinian eruption of La Virgen volcano, Baja California, Mexico

A plinian eruption occurred approximately 6500 yr ago at La Virgen Volcano, the youngest volcano of the Tres Virgenes Volcanic Complex (TVVC), located in Baja California, Mexico. Deposits of the eruption suggest a sequence of events that started with the opening of the volcanic conduit, and development of a plinian eruption column up to 18 km in height. This eruption column produced a fallout deposit with a dispersal axis toward the southwest, an areal extent of about 500 km 2 , and a minimum volume of 1.14 km 3 . Vulcanian activity (hydromagmatic) followed the plinian phase, producing pyroclastic surge and fallout deposits. The eruptive activity ceased after a basaltic-andesite lava flow was emplaced closing the eruptive activity. Petrological and geochemical evidence indicates that the eruption was triggered by magma mixing processes. Our studies confirm that La Virgen is a dormant volcano with the potential for future violent eruptions. The present study provides important information for the construction of a volcanic hazards map. Significant hazards are presented to the population living within a distance of 30 km from the volcano, together with the interstate road connecting the entire peninsula of Baja California, which runs at a distance of only 3 km from the volcano.

The 26 May 1982 breakout flows derived from failure of a volcanic dam at El Chichón, Chiapas, Mexico

The eruptions of El Chichon between 28 March and 4 April 1982 produced a variety of pyroclastic deposits. The climactic phase, on 3 April at 07:35 (4 April at 01:35 GMT), destroyed the central andesitic dome and fed pyroclastic surges and flows that dammed nearby drainages, including the Magdalena River. By late April, a lake had formed, 4 km long and 300–400 m wide, containing a volume of 26 × 10 6 m 3 of hot water. At 01:30 on 26 May, the pyroclastic dam was breached and surges of sediment and hot water soon inundated the town of Ostuacan, 10 km downstream. This hot flood was finally contained at Penitas Hydroelectric Dam, 35 km downstream, where one fatality occurred and three workers were badly scalded. Stratigraphic and sedimentologic evidence indicates that the rapidly draining lake initially discharged two debris flows, followed by five smaller debris flows and water surges. The main debris flows became diluted with distance, and by the time they reached Ostuacan, they merged into a single hyperconcentrated flow with a sediment concentration of ∼30 vol%. Deposits from this hyperconcentrated flow were emplaced for 15 km, as far as the confluence with another river, the Mas-Pac, below which the flow was diluted to sediment-laden streamflow. The minimum volume of the breakout-flow deposits is estimated at 17 × 10 6 m 3 . From high-water marks, flow profiles, and simulations utilizing the DAMBRK code from the National Weather Service, we calculated a maximum peak discharge of 11,000 m 3 /s at the breach; this maximum peak discharge occurred 1 h after initial breaching. The calculations indicated that ∼2 h were required to drain the lake.

Energy partition in the 1982 eruption of El Chichon volcano, Chiapas, Mexico

Abstract The 1982 eruption of El Chichon volcano, Chiapas, Mexico has been extensively investigated from various standpoints, including seismicity, tephra geology, atmospheric effects and tectonic structure. In this paper, the six largest of a series of explosions are isolated, considering various sources of information. Then, the seismic energy released by volcanic earthquakes and tremor is estimated using seismograms of low magnification in order to avoid saturation of seismic wave amplitudes. The explosion kinetic energy is evaluated by two methods: one by the initial velocity of the ejecta and the other by the air wave data recorded at a distance. Both the results are roughly coincident. The thermal energy is classified into two components: that transferred by volcanic ejecta and that consumed by convective clouds. The thermal energy released by the explosions is very large compared with that of mechanical energy, such as the seismic energy and explosion kinetic energy. After the last major explosion of April 4, A-type volcanic earthquakes swarmed till the end of April. Their occurrence rate decreased with time in an aftershock-like sequence. These earthquake swarms may have been due to post-eruptive readjustment of the stress field within and near the volcano. Such seismic activity contrasts with other examples of similar major eruptions, which were followed by growth of lava domes in the new craters after the paroxysmal eruptions. The 1982 eruption of El Chichon volcano has not been followed by the growth of a lava dome within the newly formed crater.

Late Holocene Peléan-style eruption at Tacaná volcano, Mexico and Guatemala: Past, present, and future hazards

Tacana volcano, located on the border between Mexico and Guatemala, marks the northern extent of the Central American volcanic chain. Composed of three volcanic structures, it is a volcanic complex that has had periodic explosive eruptions for at least the past 40 k.y. The most recent major eruption occurred at the San Antonio volcano, the youngest volcanic edifice forming the complex, about 1950 yr ago. The Pelean style eruption, issued from the southwest part of the dome, and swept a 30° sector with a hot block and ash flow that traveled about 14 km along the Cahoacan ravine. Deposits from this event are well exposed around the town of Mixcun and were therefore given the name of that town, the Mixcun flow deposit. The Mixcun flow deposit is, in the channel facies, a light gray, massive, thick (>10 m), matrix-supported unit with dispersed lithic clasts of gravel to boulder size, divisible in some sections into a variable number of flow units. The overbank facies is represented by a thin (<1 m), massive, matrix-supported unit. In both of these facies the deposit has disseminated charcoal, fumarolic pipes, and juvenile lithics with cooling joints. The Mixcun flow deposit contains clasts of (1) light gray, dense andesite, (2) dark gray, glassy and banded andesite, and (3) minor altered red andesite from the edifice, set in a matrix of sand and silt. The Mixcun flow deposit covers an area of at least 25 km2 and has a minimum estimated volume of 0.12 km3. Basaltic-andesite inclusions (54% SiO2) and various signs of disequilibrium in the mineral assemblage of the two-pyroxene andesitic products (60%–63% SiO2) suggest that magma mixing may have triggered the eruption. Following deposition of the Mixcun flow deposit andesitic to dacitic (62%–64% SiO2) lava flows were extruded and a dacitic dome (64.4% SiO2) at the San Antonio summit formed. Syn-eruptive and posteruptive lahars flooded the main drainages of the Cahoacan and Izapa-Mixcun valleys in the area of the present city of Tapachula (population 250000) and the pre-Hispanic center of Izapa. Three radiocarbon ages date this event between A.D. 25 and 72 (range ±1σ, 38 B.C.–A.D. 216), which correlates with a halt in construction at Izapa (Hato phase of ca. 50 B.C.–A.D. 100), probably due to temporary abandonment of the city caused by lahars. Another similar event would produce extensive damage to the towns (population of about 68,000 people) now built upon the Mixcun flow deposit. The main summit of Tacana volcano continues to show signs of fumarolic activity; the most recent period of activity in 1985–1986 culminated in a minor phreatic explosion.

The July 1994 episode of seismic activity at Colima Volcano, Mexico

Abstract A new period of seismic activity that culminated in a small phreatic explosion took place in Colima Volcano (Western Mexico) during the month of July 1994. In this note, we present our analysis of this seismicity based upon information from RESCO, the seismic network of the University of Colima. The activity began with a seismic swarm of type A (tectonic-like) earthquakes with epicenters towards the SSW of the summit, followed by shallow low-frequency events underneath the volcanic edifice. The activity was accompanied by landslides and culminated with an explosion that produced small ash falls on the surrounding area. The seismic activity ceased after this episode.

The origin of the recent (2012–2016) seismic activity in the Guadalajara, Jalisco, Mexico, area: A block boundary interaction?

The central part of Jalisco, Mexico, has experienced low-magnitude earthquake sequences and swarms. Although the effects of these earthquakes have been limited to relatively small areas, the earthquakes have caused general alarm among the population and, in some cases, have been catastrophic. These earthquake swarms are significant because they affect the most populous area of the state, including the capital city of Guadalajara. An extraordinary example is an earthquake swarm that started on 8 May 1912 and lasted until September of that year. The region remained seismically quiescent until May 2012, when seismic activity resumed, lasting to the present. We analyze the recent seismic activity, starting with the earthquake of 18 May 2012 (03:07 UT) at the western edge of Lake Chapala and ending with the magnitude 4.2 earthquake on 3 November 2016. Our analysis includes eight earthquakes with magnitudes between 3.5 and 4.8, the revision of hypocenter locations, and the determination of focal mechanism solutions using the inversion of the moment tensor method. When possible, inversion solutions are compared with solutions obtained with the first arrival polarity method. We compare our results for the recent seismicity with the distribution of reported damage associated with historical earthquakes. Our work indicates a N-S trending seismic source zone and an orientation of nodal planes that suggests reactivation of preexisting local faults induced by the interaction of the western border of the Trans-Mexican Volcanic Belt with the eastern border of the Jalisco Block.